# CPI (Consumer Price Index)

Hi,
I'm sure there is a simple answer to this question, but I can't seem how to figure out how the CPI is calculated on a monthly basis from the the CPI tables.
For instance, the annual increase in CPI in November 2012 is given as 2.7% (as it was in October).
Using the CPI tables (using values based on 100 as of May 2005), the index in November was 124.4 and 124.2 in October.
(see:
I'm just a little confused about how these tables relate the published annual CPI inflation on a month-to-month basis.
Thanks Rgds Neil.
>
The percentage quoted is an annual inflation rate. To get to the stated figure, you have to divide the current month's CPI index figure by that of a year previously.
In message , N. Sloane writes
The 'headline' figure is quoted as month on the same month of the preceding year, so November 2012/November 2011. Not always a sensible way to do it, but usually better than just taking one month's change.
I'm with you at a point, but if the rate for November 2012 is 124.4 and the November 2011 figure is 121.2, do I just do a literal division ?
That would give an answer of 1.02644. Assume it is 2.644%, is the just rounded upwards to the nearest digit to give the rate of 2.7% ?
Thanks Neil.
It shouldn't be. What happens is that the CPI figures have been rounded to the nearest .1 and that gives rise to the occasional 0.1% error in the anual percentage rate.
In message , Norman Wells writes
dex.html
This will give you less rounded figures than the source you are using.
Can you point me to the figures you have in mind please? The few times I have looked at the data in the ref tables they seemed all to be rounded as in the press releases etc rather than more precise data or computed from formulae - eg in
But I may well be missing soemthing there or missing another spurce altogether.
In message , Robin writes
You get one decimal place in the tables there (e.g. tables 1 or 2 in the reference you quoted) , which is more than the original poster was quoting. If you need more, you could contact them, but I don't know if they would want to publish any more detail.
Hi Sheila,
I have looked at the ONS figures and downloaded the Reference Tables. However, they are exactly the figures as the other source I was using.
So I'm still a bit confused how you can divide 124.4 by 121.2 and come out with a figure of 2.7%, because no matter how I try and round it, it always comes to 2.6%.
Neil.

If the actual figures were, say, 124.44 and 121.16, they would be rounded to 124.4 and 121.2. Dividing the more accurate figures gives a rounded 2.7%.
I am as puzzled as the OP who I see has already pointed out he quoted the same figures as in the tables.
FTAOD I expect it is indeed just a matter of the figures being rounded individually. And I am surprised there is not a prominent note to that effect.
In message , Robin writes
The obvious solution is on the ONS site, not here:
Contact
Philip Gooding
Prices
snipped-for-privacy@ons.gsi.gov.uk
Telephone: +44 (0) 1633 456900
I've contacted them. I'll be interested in their reply.
I rather doubt the ONS will tell me why you stated the tables give more decimal places than the OP quoted :)
Hi,